This invention relates to the field of phase-locked loops, and more particularly, this invention relates to the field of phase-locked loops used in communication systems that incorporate a quadrature modulator.
Chirp modulation and detection techniques are used in various devices and systems, such as tracking systems. One type of Wide Area Tracking and Location (WATL) transmitter system requires a low cost, reliable method of generating a xe2x80x9ccleanxe2x80x9d four MHz chirp in the 900 MHz band. The resulting modulation technique could be used in a small, battery operated transmitter used for tracking people. An example of chirp modulation used in location determination is disclosed in commonly assigned U.S. Pat. No. 5,859,613 to Otto, the disclosure which is hereby incorporated by reference in its entirety.
In one proposed design, a read only memory (ROM) pattern generated a sequence of 90 degree phase shifts causing a linear FM modulation of 4 MHz over a full sequence of 8192 microseconds. Continuous xe2x88x9290 degree shifts occurred at 8 MHz and created a xe2x88x922 MHz offset of the voltage controlled oscillator (VCO). The details of data encoding the chirps actually required going from less than xe2x88x922 MHz, depending on data value, up to +2 MHz. One possible improvement of this design was to use a step phase accumulator in place of the ROM and implement this along with the sequencer in a small, high speed, programmable Application Specific Integrated Circuit (pASIC).
However, current FCC requirements are stringent on xe2x80x9cout-of-bandxe2x80x9d spurs. This would result in requiring an expensive and custom SAW filter. Another expensive and possible approach would be to generate the chirps at intermediate frequencies (IF) using high speed digital signal processing (DSP) and then up-converting. In one approach, it was possible to generate a scaled down chirp and use it as a reference to the phase-locked loop (PLL) resulting in the VCO outputting the required chirp at the desired carrier. This approach would have the advantage of allowing the loop filter, already required, to reduce the out-of-and band modulation components. No costly filtering would be needed. However, it had the disadvantage of requiring generation of a very precise scaled down chirp. This would still require an expensive DSP engine.
U.S. Pat. No. 5,313,173 to Lampe discloses a phase-locked loop that incorporates a quadrature modulator for generating a constant envelope phase or frequency modulation. The system does provide for suppression of undesired amplitude modulation (AM) and phase modulation (PM) components of the modulated signal. However, chirp generation is desired in the Wide Area Tacking and Location system described above. These chirps would require data encoding and a reference, which cannot be provided by the quadrature modulated phase-locked loop system as disclosed in the ""173 patent.
It is therefore an object of the present invention to provide a method of generating a modulated chirp signal that has data and reference, and can be used in a Wide Area Tracking and Location system, such as in a small, battery operated, transmitter.
The method, in accordance with the present invention, generates a modulated chirp signal and comprises the steps of generating a phase-locked loop output signal and sampled feedback signal from a voltage controlled oscillator. The method also comprises the step of receiving the sampled feedback signal within a quadrature I/Q phase modulator and generating I/Q quadrature signals from an I/Q generator circuit to produce a desired modulation of the sampled feedback signal as a string of +/xe2x88x9290 degrees phase shifts, creating a desired offset at the voltage controlled oscillator. The method also comprises the step of chirp modulating the I/Q quadrature signals and the resultant phase-locked loop output signal by a chirp signal comprising a sequence of chirps having a reference chirp followed by a plurality of data chirps.
In still another aspect of the present invention, each data chirp encodes a plurality of n-bits as an offset to a starting frequency. Each data chip can be correlated by two (Di+1) microseconds later than had it been a reference chirp.
In still another aspect of the present invention, the method comprises the step of data encoding the chirp from less than about xe2x88x922 MHz to about +2 MHz. Out-of-band spurs can be reduced by a loop filter that receives signals from a phase detector. The step of generating the chirp signal can include the step of generating from a chirp controller. The frequency at the phase detector is about one MHz and the I/Q phase modulator output is about 907 MHz. The I/Q generator circuit comprises a programmable Application Specific Integrated Circuit (pASIC). The chirp signal can comprise a reference chirp followed by five data chirps.
A system of the present invention generates a modulated chirp signal and includes a phase-locked loop circuit comprising a voltage controlled oscillator, a phase detector, a quadrature I/Q phase modulator and a feedback loop for generating a phase-locked loop output signal from the voltage controlled oscillator and a sampled feedback signal from the voltage controlled oscillator to the quadrature I/Q phase modulator. An I/Q generator circuit is operatively connected to the quadrature I/Q phase modulator for generating I/Q quadrature signals to the quadrature I/Q phase modulator and producing a modulated signal as a string of +/xe2x88x9290 degrees phase shifts to create a desired offset at the voltage controlled oscillator. A chirp controller is connected to the I/Q generator circuit for generating a chirp signal to the I/Q generator circuit comprising a sequence of chirps having a reference chirp followed by a plurality of data chirps.
The chirp controller also data encodes the chirps from less than about xe2x88x922 MHz to about +2 MHz. The loop filter reduces out-of-band spurs and the phase detector is about 1 MHz and the I/Q phase modulator is about 907 MHz.